Gantry cranes are adapted for lifting and moving a large, heavy load such as a freight container. A gantry crane comprises a hoist mounted on a hoist carriage able to move laterally along one or more crossbeams. Two or more vertical supports, each attached at an upper end to an end of a crossbeam, hold the crossbeam above the ground. Some gantry cranes have wheels and motors coupled to structural members attached to the lower ends of the vertical supports, enabling the gantry crane to traverse under power from a location where a load is to be picked up to another location where the load is to be set down.
Mobile gantry cranes as used in freight yards generally have a pair of parallel crossbeams for supporting the hoist and hoist carriage, at least four vertical supports, and wheels near each lower corner of the crane. Some gantry cranes known as rail-mount gantry cranes (RMG crane) have flanged metal wheels adapted for rolling on track rails fixed to the ground, similar to wheels and rails used by trains. Rails enable an RMG crane to move quickly and precisely, but the cost of installing or relocating rails is high. Another type of gantry crane known as a rubber tire gantry crane (RTG crane) has rubber tires instead of metal wheels and is not constrained to operate from fixed rails. Instead, the RTG crane wheels move on a prepared surface known as a runway. A pair of parallel runways comprises a lane along which the RTG crane moves. RTG crane wheels may be steerable for changing a direction of travel of the RTG crane, for example to move from one lane to another. RTG cranes are sometimes preferred over RMG cranes where logistical flexibility is desirable.
Some RTG cranes are powered by an onboard diesel engine coupled to an electrical generator. Electrical output from the generator supplies power to electric motors for moving the RTG crane and operating the hoist and other equipment. However, emissions from diesel engines are becoming a matter of concern in some ports and terminal areas. An alternative is to operate an RTG crane from electrical power supplied from an electrical distribution network in the terminal area. Electrical distribution networks are sometimes referred to as shore power when the network is close to a loading and unloading area for cargo ships.
An example of an RTG crane operated from shore power is shown in FIG. 1. An RTG crane 100 representative of an RTG crane known in the art comprises a hoist 102 and rubber tires 104. The RTG crane 100 is pictured moving along runways 112. Two parallel runways 112 comprise an RTG lane. A runway 112 may comprise reinforced concrete pavement having sufficient strength to bear the weight of a heavily-loaded RTG crane. The RTG crane 100 straddles a load, lifts it, and transports it along an RTG lane. In the example of FIG. 1, one of a stack of freight containers 114 is an example of a load to be lifted and transported by the RTG crane 100. One of the freight containers in FIG. 1 has a width dimension of approximately 8 feet (2.4 meters), a height dimension of approximately 8 feet 6 inches (2.6 meters), a length of approximately 20 feet (6.1 meters), and may weigh 20 tons (approximately 18,000 kilograms) or more. Freight containers having other dimensions and maximum weights are also moved by RTG cranes. Some RTG cranes are capable of lifting much larger loads than a single freight container.
A substantial amount of electrical power may be required to operate an RTG crane. The RTG crane 100 of FIG. 1 receives electrical power through a cable connected to shore power. For reasons of transmission efficiency, power may be transmitted through the cable at a high voltage and a relatively low current. Transmitting power at a relatively low current allows the use of a cable having a smaller diameter and less stiffness than a cable carrying high current. For example, input power for some RTG cranes is an alternating current (AC) with a voltage in a range from about 2000 volts AC (VAC) to about 6000 VAC. To avoid damaging the cable and exposing personnel and equipment to high voltage, the cable carrying RTG crane input power may be placed in a trench next to a runway. Fences or other barriers may also be installed to further protect personnel, the cable, and other equipment.
High voltage cable is wound and unwound from a cable reel on the RTG crane 100 of FIG. 1 as the crane traverses along an RTG lane. In FIG. 1, a crane-mounted cable reel 106 is attached to a side of the RTG crane 100. A high voltage cable 108 is lifted from a cable trench 110 and wound around the crane-mounted cable reel 106 as the RTG crane 100 moves along an RTG lane. When the RTG crane 100 moves in the opposite direction along the lane, the high voltage cable 108 is unwound from the crane-mounted cable reel 106 and placed back into the cable trench 110. A rotary power coupler at the hub of the crane-mounted cable reel 106 connects high voltage from the high voltage cable 108 to the input side of a high voltage transformer 116. The output of the high voltage transformer 116 is a relatively low AC voltage, for example an AC voltage in a range from about 400 VAC to about 500 VAC. The transformer output voltage is connected to an electrical input on the RTG crane to operate electric motors and other equipment.
Some freight terminals have more than one RTG lane to enable an RTG crane to access multiple loading and unloading locations. An example of a terminal area having more than one RTG lane is shown in FIG. 2. In FIG. 2, a first load location A and a second load location B are serviced by an RTG crane 100 moving along a first RTG lane comprising two runways 112-1. A second pair of runways 112-2 forming a cross lane provide access for the RTG crane 100 to a third load location C, which is approached on a third RTG lane comprising two runways 112-3. A first cable trench 110-1 holds a high voltage cable that is connected to the RTG crane 100 when the RTG crane is traveling between locations A and B. A second cable trench 110-2 holds a high voltage cable that is connected to the RTG crane 100 when the RTG crane is moving along the RTG lane that straddles load location C.
To move cargo from load location A to load location C in the example of FIG. 2, the RTG crane 100 performs a cross-lane maneuver by advancing on runways 112-1 from location A to a position near cross lane runways 112-2. The high voltage cable from the first cable trench 110-1 is disconnected from the RTG crane 100 and a mobile source of high voltage electrical power 118, for example a truck-mounted electrical generator, is connected by a cable to the RTG crane 100. The RTG crane 100 turns onto the cross lane and moves near the RTG lane comprising the runways 112-3 for load location C. The RTG crane 100 is guided onto the RTG lane for load location C, the mobile high voltage source 118 is disconnected from the RTG crane, and the high voltage cable in the second cable trench 110-2 is attached to the RTG crane. Finally, the RTG crane 100 proceeds to load location C and unloads its cargo there.
In the cross-lane maneuver described above, there are two high voltage disconnection steps and two high voltage connection steps as the RTG crane 100 moves from shore power to a mobile power generator to shore power again while changing lanes. Because of safety hazards associated with high voltage, at some freight terminals high voltage cables may be connected and disconnected only by specially trained personnel. Cargo operations may be slowed if trained personnel are not available at the time and place a cross-lane maneuver is to be performed. Cargo operations may further be slowed by the time needed to make and unmake high voltage connections.
It is preferred that the RTG crane 100 be guided so that the high voltage cable 108 is pulled from or placed into the cable trench 110 with a minimum of stress and wear on the cable and crane-mounted cable reel 106. It is further preferred that the high voltage cable 108 is laid flat and straight in the bottom of the cable trench 110. For example, some high voltage supply systems require an RTG crane 100 to deviate from a selected line of travel by a distance of less than about ±0.4 inch (±10 mm) to achieve a selected high voltage cable placement. However, an RTG crane 100 having a common type of steering system meanders about 6 inches (150 mm) on either side of a selected line of travel as the RTG crane 100 traverses along a runway. Furthermore, it is well known that some human operators achieve a guiding accuracy of about ±10 inches (254 millimeters) while driving diesel-powered RTG cranes.
In the example above, the combination of RTG crane 100 position errors from the steering system and from operator ability may exceed the positioning accuracy specification for cable placement. Accurate positioning of the RTG crane 100 relative to a cable trench 110 may therefore require specialized and costly position measurement and control equipment. Position measurement and control equipment for accurately guiding an RTG crane 100 along a selected path is known as a straight-steering system. Even for an RTG crane 100 equipped with an accurate straight-steering system, limitations in the speed with which position measurements are made and the speed and accuracy with which position errors are corrected may cause the RTG crane 100 to be operated at a speed lower than preferred for economical cargo operations.
What is needed is a means of connecting electrical power to an RTG crane without connecting and disconnecting high voltage cables during routine cargo transfer operations. What is also needed is a means of accurately guiding an RTG crane to achieve a selected high voltage cable placement without requiring an RTG crane straight-steering system.